Filament-wound pressure vessel

ABSTRACT

A filament-wound pressure vessel constructed of a rigid composite of an epoxy resin matrix reinforced with continuous filaments of a p-aramid coated with an adhesion modifier.

RELATED APPLICATION

This application is a divisional of our application Ser. No. 739,861,filed May 31, 1985 U.S. Pat. No. 4,678,821.

BACKGROUND OF THE INVENTION

Mumford et al. described the use of "Kevlar 49" in filament woundpressure vessels in a note from the AIAA/SAE/ASME 18th Joint PropulsionConference, June 21-23, 1982, Cleveland, Ohio. For such application thefilaments are ordinarily embedded in an epoxy resin matrix. Mumford etal. report that the fiber stress performance in the filament woundpressure vessel is lower than the nominal strand tensile strength. Oneway suggested in the article for utilization of a higher fraction of thefiber ultimate strength is coating the fiber with from 5 to 9% of asilicone release agent before application of the epoxy resin. Thissystem has deficiencies and is not employed in polar windings of thepressure vessels. The present invention overcomes certain of thesedeficiencies and provides unusually high impact resistance to pressurevessels.

SUMMARY OF THE INVENTION

A rigid composite comprising an epoxy resin matrix reinforced withcontinuous p-aramid filaments coated with from about 0.2 to 2 percent byweight of a solid adhesion modifier, said coated filaments when embeddedin an epoxy matrix and tested in accordance with ASTM D2344-7.6 exhibita short beam shear strength (SBSS) for the composite of between about2.5 and 5.5 Kpsi. Preferably the adhesion modifier is a2-perfluoroalkylether ester or paraffin wax. Generally from about 20 to80 percent by volume of filaments is present based on the weight of thecomposite.

DETAILED DESCRIPTION OF THE INVENTION

This invention relates to continuous filament reinforced, rigidcomposites. High tenacity, high modulus p-aramid filaments are usefulfor such composites.

The term "aramid" is used to designate wholly aromatic polyamides. Notall aramid fibers are useful in the present invention but only thosederived from aromatic polyamides whose chain extending bonds are eithercoaxial or parallel and oppositely directed. Minor amounts of unitswhose chain extending bonds are not oppositely directed, e.g.,meta-oriented bonds, may be present provided the filament tenacity andmodulus are not unduly reduced. High strength, high modulus aramidfibers useful in the present invention may be prepared by the processesdescribed in U.S. Pat. Nos. 3,767,756, 3,869,430 and 4,075,172. Thefibers are characterized by filament tenacities of at least 15 gpd (13.2dN/tex) and moduli of at least 400 gpd (354 dN/tex). These fibers willbe referred to hereinafter as p-aramid fibers. Particularly preferredare p-aramid fibers based on poly(p-phenylene terephthalamide) asproduced by Du Pont under the trademarks Kevlar®.

The matrix component of the composite of the invention is an epoxyresin. Such materials are well known in the art and in the cured stateprovide strong rigid structures. Among suitable epoxy resins, there maybe mentioned those from epichlorohydrin and bisphenol A. In thecomposites of the invention the p-aramid filaments are usually alignedwithin the epoxy resin matrix in the direction in which stress is to beapplied. The filaments generally comprise from 20 to 80 percent byvolume of the composite preferably from 60 to 80 percent by volume.

The advantages of the present invention are seen when the filamentsbefore embedment have been coated with an adhesion modifier. Thefunction of the modifier is to reduce the adhesion level between thep-aramid filaments and the epoxy matrix as discussed in greater detailbelow. From about 0.2 to 2 percent by weight of such modifier is appliedto the filaments to achieve the desired SBSS value of between about 2.5and 5.5 Kpsi. It may be applied in any of a variety of ways to provideeven distribution on the surfaces of the filaments. For example, it maybe dissolved in a solvent, applied to the filaments and the solventdriven off, or it may be dispersed in a carrier (e.g., water) andapplied to the filaments, or it may be applied directly in the absenceof other ingredients. One of the more common ways of application is froma dispersion of the adhesion modified in water. In a generally usefulway of preparing such a dispersion, the adhesion modifier is melted ordissolved in an organic solvent (e.g., methylisobutyl ketone) and mixedwith an aqueous solution of the dispersing agent, yielding a two-phasemixture. The mixture is stirred under high shear (e.g., in a Waringblender) to yield a dispersion. The organic solvent is stripped off bydistillation under vacuum; the residual mixture is the desired aqueousdispersion. Suitable dispersing agents are "Armeen" 14D and 18D,"Merpol" OJS, "Arquad" 1250, "Merpol" HC5, sodiumdodecylbenzenesulfonate, "Avitex" DN-100, and octadecyltrimethylammonium chloride.

The adhesion modifier may be characterized as a solid which is notreadily absorbed or dissolved in the filaments or resin. As a solid itwill have less tendency to migrate during preparation of the compositeor in use. It is important that the modifier reduce the adhesion betweenfiber and matrix to limit transverse loading which tends to preventmaximum utilization of ultimate fiber strength. On the other hand thereduction in adhesion should not be so great that the load of brokenfilaments cannot be redistributed through the matrix polymer to otherfilaments. With the composites under consideration it has been foundthat the desirable qualities are achieved if at the low modifier add-onlevel specified, i.e., between about 0.2% to 2% by wt., the short beamshear strength is between 2.5 and 5.5 Kpsi (thousand pounds per squareinch) when a unidirectional flat specimen is measured in accordance withASTM D2344-76.

One advantage of obtaining the desired adhesion values at low add-onlevels is that there is overall weight reduction of the composite ascompared to those amounts of silicone materials reported in the art.Another is that low add-on permits a greater concentration of filamentsbased on the weight of the composite. Two groups of surface modifiersfor p-aramid filaments useful for this invention are certainfluorochemicals and paraffin waxes. The fluorochemicals may be describedas 2-perfluoroalkylethyl esters where the 2-perfluoroalkylethyl grouphas the structure C_(n) F_(2n+1) (CH₂)₂ - and n=5-11. The estersexemplified herein are those of methacrylic acid in polymeric form,citric acid urethane, and phosphoric acid or its ammonium salts. Estersof this type are shown in U.S. Pat. Nos. 3,282,905; 3,378,609 andothers.

The present invention is particularly useful in filament wound pressurevessels. Such vessels can tolerate impact and withstand pressure betterif the reinforcing filaments have been coated in accordance with thisinvention prior to being incorporated in the epoxy resin matrix thanwhen uncoated filaments have been used. Use of these coatings alsoavoids the problems associated with application of liquid siliconerelease agents as used in the art. The latter require extensive dryingor curing periods, and are messy to use. They are generally employed inlarge amounts, i.e., 5 to 9% (see Mumford et al. supra), thus addingundesirable weight to the composite. It is also understood that thefrictional qualities of such silicone coated yarn discourage use of theyarn in polar windings of pressure vessels.

It will be understood that the examples below are believed to berepresentative of the present invention. Further, a change in the amountof add-on might bring the short beam shear strength value outside of thedesired range of 2.5 to 5.5 Kpsi.

EXAMPLES

Examples 1-4 are all unidirectional, filament wound composites comprisedof epoxy resin and adhesion modified p-aramid yarns. The examples differfrom each other in the nature and amount of the adhesion modifier (seeTable I). The adhesion modified yarns were prepared by combining fourends of p-aramid yarn (1,140 denier-768 filaments; Kevlar® 49manufactured and sold by E. I. du Pont de Nemours and Company; standardproducer-applied finish), dipping in an aqueous dispersion of theappropriate adhesion modifier, drying by passing through an oven at 200°C. (about 2 min. residence time) and winding on a package. The amount ofadhesion modifier applied to the yarn was controlled by diluting theaqueous dispersions of the modifier to the appropriate level.

The composites are all prepared in the same manner from the variousadhesion-modified yarns by tensioning and dipping the yarns in acomposition of 100 parts by weight of an epoxy resin (EPON 826, Shell),25 parts by weight of 1,4-butanediol diglycidyl ether (RD-2, Ciba Giegy)and 30 parts by weight of an amine curing agent (TONOX 60/40, NaugatuckChem. Co.) such that the composition was 60 volume percent fiber andwinding the wet yarn on a rectangular mold.

The rectangular mold had two cavities 6" long×1/2" wide×1/2" deep andresin coated yarn was wound into these cavities. Two cover plates werelaid on the cavities and screws were partially tightened to hold thecover plates to the mold. Four shims (1/8" thick) were placed betweenthe mold and the cover plates to give desirable sample thickness.

The complete mold was then placed in a vacuum chamber at roomtemperature and 25 inch vacuum for 30 minutes to degas the resin. Allscrews were tigthened and the sample cured in two steps (120° C. for 90min. and 175° C. for 60 min.).

The cured composites were cooled down to ambient temperature. The screwsand cover plates of the molds were removed. (All metal parts weresprayed with mold release before winding for easy disassembly).

The composite was cut with a saw to give two unidirectional filamentwound flat laminate samples 6" long×1/2" wide×1/8" thick. Fiber volumeis 60±4%. All fiber was aligned in the length direction.

The shear strength of the composites was tested by the ASTM Short-BeamMethod D-2344-76 (pp. 381-384 of Annual Book of ASTM Standards, Part 36,1977 Edition). Sample size was 0.5 in. wide, about 0.125 in. thick, and3.0 in. long (span length to depth ratio of 4/1). Four specimens of eachcomposite were tested after conditioning. The short-beam shear strength(SBSS) was calculated from the breaking load and sample dimensions andis listed for each Example in Table I.

A control (Sample A) was made similarly to Example 1-4 except that noadditional adhesion modifier was employed on the yarn other than thestandard producer-applied finish.

                                      TABLE I                                     __________________________________________________________________________    Examples 1-4 and Sample A                                                                               Dispersing                                                                    Agents For     SBSS                                 Adhesion Modifier on Kevlar ® 49                                                                    Modifier % Add-On                                                                            (Kpsi)                               __________________________________________________________________________    Example                                                                       1.    a 56:100 mixture (by wt.) of a                                                                    "Armeen" 14D                                                                           0.5   4.4                                        fluorochemical mixture and a                                                                      &                                                         wax paraffin mixture. The                                                                         "Armeen" 18D                                              fluorochemical mixture was                                                    composed of two interpolymers,                                                one interpolymer comprising 57%                                               of total is derived from about                                                75% 2-perfluoroalkylethyl ester of                                            methacrylic acid where the                                                    2-perfluoroalkylethyl group has the                                           formula, C.sub.n F.sub.2n+1 (CH.sub.2).sub.2,                                 and n has a value of from 5 to 11,                                            about 25% 2-ethylhexyl methacrylate,                                          about 0.25% Nmethylolacrylamide and                                           about 0.25% 2-hydroxyethyl-methacrylate;                                      the second interpolymer, comprising                                           43% of the total, is derived from                                             about 97-98% of 2-ethylhexyl methacrylate,                                    2% Nmethylol acrylamide and 0-1%                                              ethylenedimethacrylate. The wax paraffin                                      mixture was comprised of 50% of a wax                                         paraffin derived from petroleum and                                           50% of a derivative of melamine having                                        the following formula:                                                         ##STR1##                                                                     where R is: -                                                                                      ##STR2##                                           2.    2-Perfluoroalkylethyl ester of                                                                    Sodium   0.4   4.8                                        citric acid urethane derived from                                                                 Dodecylbenzene                                            1,6 hexamethylene diisocyanate and                                                                Sulfonate                                                 tri(2-perfluoroalkylethyl) citrate                                            where the 2-perfluoroalkylethyl                                               group has the formula given in                                                Example 1.                                                              3.    A 3/1 mixture (by wt.) of wax                                                                     Octadecyl                                                                              0.9   5.1                                        paraffin derived from petroleum                                                                   Trimethyl-                                                and a copolymer of stearyl                                                                        ammonium                                                  methacrylate (70%) and                                                                            Chloride                                                  diethylaminoethyl methacrylate                                                (30%).                                                                  4.    Mixture of ammonium salts of                                                                      "Avitex" 0.3   5.1                                        mono and bis-2-perfluoroalkylethyl                                                                DN-100                                                    phosphate esters (1:1 mono                                                    to bis ratio) where 2-perfluoro-                                              alkylethyl group is the same as                                               that of Example 1.                                                      Sample A                                                                            None                --       --    7.6                                  __________________________________________________________________________

EXAMPLE 5

This Example describes the preparation of a composite in the form of afilament-wound pressure vessel. The pressure vessel was prepared andtested as described in ASTM Method D-2585-68 (Reapproved 1974),Procedure A [Annual Book of ASTM Standards (1977), Part 36].

Adhesion-modified p-aramid yarn (1140 denier, 768 filaments Kevlar® 49)was prepared by applying 1.4 weight percent (dry basis) of the modifierdescribed in Example 1 (Table I) to the yarn on the spinning machine inplace of the conventional finish. The modifier was applied as an aqueousdispersion via a kiss-roll applicator.

The sand mandrel upon which the yarn was wound to form the pressurevessel was prepared as described in ASTM Method D-2585 p. 479 and hasthe dimensions shown in FIG. 1, p. 477. The metal polar bosses wereattached to the mandrel and a release agent was sprayed onto themandrel. A polyurethane rubber liner was put on the mandrel, and themandrel was attached to a computer controlled filament winder. Theadhesion-modified yarn described above was prepreged with epoxy resin(UF3298 Thiokol; 35±2% resin based on weight of yarn) and wound on themandrel starting with two polar (helical) plies followed by three hoopplies. The polar to hoop stress ratio created was 0.85. The whole systemwas cured in an oven for 3 hrs. at 120° F. followed by 4 hrs. at 250° F.After the system cooled, water was poured into the vessel to dissolvethe mandrel binder and flush the sand and the release agent out. Thevessel was dried, weighed, and pressurized in a hydrotest confinement,and the burst pressure was measured. Five vessels were made and burst toestablish statistically reliable data. The average burst pressure was3601 psi and the average calculated PVc/Wc [Burst Pressure (psi)×Volumeof Composite (in³)/Weight of Composite (Lb.)] was 1.79×10⁶ in. Thecalculated hoop stress using netting analysis was 447 Kpsi.

Control Sample B

A control filament-wound pressure vessel was prepared similarly toExample 5 with the exception that the yarn contained no special adhesionmodifier, but only the standard commercial finish. Five vessels wereprepared and tested. The average burst pressure was 2957 psi and theaverage calculated PVc/Wc was 1.27×10⁶ in. The calculated hoop stressusing netting analysis was 367 Kpsi.

EXAMPLE 6

This example is similar to Example 5 except that the finished mandreland, hence, the pressure vessel was larger (mandrel diameter, 6.68 in.and cylinder length, 5.24 in) and the mandrel winding sequence, epoxyresin, and curing cycle were different as detailed below.

The adhesion modified yarn was prepreged with epoxy resin (LRF 216Brunswick Co.; 35±2% resin based on a weight of yarn) and wound on themandrel with the following sequence; one hoop layer, one polar layer,polar domes, one hoop layer, one polar layer, one and a half hoop layer,one polar layer, and one hoop layer. The polar-to-hoop stress ratiocreated was 0.83. The whole system was cured in an oven for 18 hrs. at125° F., 2 hrs. at 175° F. and 11 hrs. at 210° F. The mandrel wasremoved, and the pressure vessel was dried and tested as in Example 5.Four vessels were made and burst to establish statistically reliabledata. The average burst pressure was 6688 psi, the average calculatedPVc/Wc as 1.410×10⁶ in., and the calculated hoop stress using nettinganalysis was 515 Kpsi.

The average short-beam-shear-strength of flat laminates fromadhesion-modified p-aramid yarn of Examples 5 and 6 was 3.4 Kpsi whenmeasured in accordance with ASTM 2344-76. The flat laminates were madeand tested in the same manner described for Examples 1-4.

Control Sample C

A control filament-wound pressure vessel was prepared similarly toExample 6 with the exception that the yarn contained no special adhesionmodifier, but only the standard finish applied to the commercial yarn.Four pressure vessels were made and tested which gave an average burstpressure of 5032 psi, an average PVc/Wc of 1.063×10⁶ in. and acalculated hoop stress of 388 Kpsi.

What is claimed is:
 1. A filament-wound pressure vessel formed of arigid composite comprising an epoxy resin matrix reinforced withcontinuous p-aramid filaments coated with from about 0.2 to 2 percent byweight of a solid adhesion modifier selected from the group consistingof 2-perfluoroalkyl ethyl ester, paraffin wax and mixtures thereof,wherein, said coated filaments when embedded in an epoxy matrix andtested in accordance with ASTM D2344-76 exhibit a short beam shearstrength of between about 2.5 and 5.5 Kpsi.
 2. A filament-wound pressurevessel according to claim 1 wherein the adhesion modifier comprises a2-perfluoroalkylethyl ester.
 3. A filament-wound pressure vesselaccording to claim 1 wherein the adhesion modifier comprises a paraffinwax.